That HTML comment is a template for how to
automatically add text to your Free Nodelet.
The link at the bottom triggers this template
to be expanded. If you go to node 22609,
your Free Nodelet will show a 'Add "tye" above'
link. Click it and you'll return to the same
node but your Free Nodelet contents will be
changed to:

Let's break down how this works.
{<!--} starts the HTML comment
that contains the template.
{todo-} as the first part of the
comment gives the template its name, "todo".
The {<li>[id://`id`]} and
{</li>} are the body of the
template, that is, the text that gets expanded
and then inserted. {-->} closes
the HTML comment and ends the template.

{[href://} starts a link back to
PerlMonks. {?node_id=} lets us
specify which node ID we want the link to take
us to. The {`id`} gets replaced
with the current node's ID so that the link
takes us back to whatever node we were
already at. {;op=_freer} means
that clicking the link will run the '_freer'
opcode which is what adds stuff to our Free
Nodelet. {;at=todo} tells the
_freer opcode the name of the template(s) we
want expanded. {|Add]} finishes
the link and says that the word "Add" is what
we'll see and can click on to activate the
link.

{"} and {" above.}
are just text to be displayed in the nodelet.
{`title&`} gets replaced with the
title of the current node but with special
HTML characters encoded as entities such that
the title displays correctly. So what we see
in the nodelet looks like:

When we click that link, our browser goes to
node 22609 (again) and the _freer opcode
gets run. It finds the "todo" template and
pulls out the body of it. It expands the
{`id`}, replacing it with the
current node's ID (22609), and then inserts
the resulting expanded template body right
before the template in our Free Nodelet's
contents.

Then the node gets displayed to the browser
with the newly updated Free Nodelet.